This invention relates generally to nuclear reactors and more particularly, to apparatus for securing jet pump assemblies within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The inlet mixers are each held in place at the top end by a preloaded beam. To secure the assembly, the jet pump beam is assembled with a high preload, applied by installing the jet pump beam bolt with a hydraulic tensioner.
High static and dynamic jet pump flow loads on the inlet mixer can, under some conditions such as insufficient beam preload, cause oscillating motion and detrimental vibration excitation in the jet pump. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue. Extreme component degradation can require plant shutdown. To assure the required preload is maintained, the beam bolt is securely locked to prevent loosening after tensioning is completed. The locking device design and engagement actuation must not interfere with operation of the tensioner. Typically, the beam bolt is locked by tack welding a sleeve lock, or keeper, which mates with the bolt hex drive, to a lock plate attached to the beam. This arrangement requires complex welding equipment which must function in the reactor under about 60 feet of water. The tack welding must be performed each time the jet pump is disassembled for cleaning or other maintenance, which consumes time on the limited reactor servicing critical path schedule. Additional delays can also occur due to the difficulty of performing the welding in the reactor with consistent quality. The keeper tack welds are broken each time the jet pump is disassembled, and the keeper can only be reused a few times before requiring replacement. This requires additional outage maintenance time.
It would be desirable to provide a mechanical locking device to replace the tack-welded keeper beam bolt locking arrangement. It would also be desirable to provide a mechanical locking device that facilitates assembly and disassembly of the jet pump.
In an exemplary embodiment, a locking device for a jet pump beam, includes a locking sleeve sized to receive a jet pump beam bolt The locking sleeve includes a base portion having an upper surface, and a plurality of ratchet teeth extending around the periphery of the base portion. The locking device further includes a lock plate having a beam bolt opening, and an integral beam spring arm. The beam bolt opening is sized to receive the locking sleeve. The spring arm includes a plurality of ratchet teeth extending from at least a portion of a side of the spring arm facing the center of the beam bolt opening. The spring arm ratchet teeth are sized to mesh with the locking spring ratchet teeth.
The lock plate also includes a lip extending at least partially around the beam bolt opening, and the lip is sized to engage the upper surface of the locking sleeve base portion. Further, the lock plate includes at least one guide pin opening sized to receive a jet pump beam guide pin.
The spring arm includes a notch adjacent the ratchet teeth. The notch is sized to receive a detent extending from the lock plate. The spring arm is movable between a first position where the detent is positioned in the notch and the spring arm ratchet teeth are engaged with the locking sleeve ratchet teeth, and a second position where the side of the spring arm facing the center of the beam bolt opening is in contact with the detent and the spring arm ratchet teeth are disengaged from the locking sleeve ratchet teeth.
The above described locking device replace the tack-welded keeper beam bolt locking arrangement of known jet pumps. Also, the above described locking device is reusable and facilitates simplified assembly and disassembly of the jet pump with a minimum of labor and equipment.